Cerebrovascular responses during lower body negative pressure-induced presyncope

Background: Reduced orthostatic tolerance is commonly observed after spacef light, occasionally causing presyncopal symptoms which may be due to low ce rebral blood flow (CBF). It has been suggested that CBF decreases in early stages of exposure to orthostatic stress. The purpose of this study was to investigate cerebrovascular responses during presyncope induced by lower bo dy negative pressure (LBNP). Hypothesis: Although CBF decreases during LBNP exposure, blood pressure (BP) or heart rate (HR) contributes more to induc e presyncopal conditions. Methods: Eight healthy male volunteers were expos ed to LBNP in steps of 10 mm Hg every 3 min until presyncopal symptoms were detected. Electrocardiogram (ECG) was monitored continuously and arterial BP was measured by arterial tonometry. CBF velocity at the middle cerebral artery was measured by transcranial Doppler sonography (TCD). Cerebral tiss ue oxygenation was detected using near-infrared spectroscopy (NIRS). We foc used our investigation on the data obtained during the final 2 min before t he presyncopal endpoint. Results: BP gradually decreased from 2 min to 10 s before the endpoint and fell more rapidly during the final 10 s. HR did no t change significantly during presyncope. CBF velocity did not change signi ficantly, while cerebral tissue oxygenation decreased prior to the presynco pal endpoint in concert with BP. Our results suggest that CBF is maintained in the middle cerebral artery during presyncope, while BP decreases rapidl y. Conclusions: Cerebrovascular hemodynamics are relatively well maintained while arterial hypotension occurs just prior to syncope.